Solar-to-electricity conversion efficiency is the key technical driver influencing photovoltaic (PV) module costs. The record efficiency of crystalline silicon (c-Si) single-junction PV devices increased from 25% to 25.6% during the last fifteen years, asymptotically approaching the Shockley-Queisser (S-Q) efficiency limit of 29.4%. To exceed this efficiency limit, multi-junction solar cells have been proposed.
Multi-junction (MJ) solar cells have multiple p-n junctions made of different semiconductor materials. The p-n junction of each different semiconductor material produces electric current in response to different wavelengths of light. The use of multiple semiconducting materials allows the absorbance of a broader range of wavelengths, improving the cell's solar-to-electricity conversion efficiency.
Currently, high-efficiency commercial “tandem” solar cells (two p-n junctions or “absorbers”) are exceedingly expensive, as they are made by growing films of III-V semiconductors epitaxially on single-crystal wafers. Tandem solar cells have also been made with CIGS, amorphous silicon and organic semiconductors using low-cost processing methods, but these have achieved only modest record cell efficiencies of 23.2, 13.4, and 12.0 percent, respectively.
It was previously noted that PV module efficiency is main driver for reducing PV module prices. Yet, the efficiency of a solar cell is fundamentally limited by its energy band gap (Eg). Silicon, the most widely used material for solar cells, has an Eg of 1.12 eV, which limits its Shockley-Queisser efficiency to about 29 percent. Because advanced solar-cell technology already approaches the practical theoretical efficiency limit, the maximum price savings available by further efficiency improvements is finite. To continue extracting cost-reduction benefit from efficiency, concepts beyond the S-Q efficiency limit are needed.